1. Technical Field
In general, the invention relates to the adjusting of the output features of a switched-mode power supply by means of a given control arrangement. In particular, the invention relates to a certain simple structure of the primary side of the switched-mode power supply, which structure helps to keep the secondary side voltage and current within desired limits.
2. Discussion of Related Art
Switched-mode power supplies are generally used in applications where the output voltage and output current of a device generating direct current should be controlled in an accurate and versatile manner. The principle of a switched-mode power supply is the feeding of electric power impulses to an inductive component, which has an energy-storing magnetic field. With a suitable discharge coupling, the stored energy is discharged into output voltage and output current, the values of which can be adjusted by altering the properties of the impulse supply. Electric safety often requires that there is no galvanic connection between the input and output pails of the switched-mode power supply. A common practice in this case is to use a isolation transformer, which in its simplest form consists of one primary coil and one secondary coil. The transformer divides the power supply into a primary side and a secondary side, so that the primary side connections feed electric power into the primary coil, the magnetic field of the transformer transmits the power to the secondary coil, and the secondary side connections discharge the power from the secondary coil into output voltage and output current.
The simplest adjusting method that can be used for affecting the value of the output voltage and output current of a galvanically isolated switched-mode power supply is a so-called fixed power adjustment, where the primary side always feeds the same electric power to the isolation transformer. With a sufficient accuracy it can be assumed that now also the output power of the power supply is constant. On the other hand, the output power is the product of the output current and output voltage, and therefore in a fixed power adjustment, the output voltage and output current are inversely proportional to each other, i.e. as the output voltage decreases, the output current grows, and vice versa. In more complicated adjusting systems, on the secondary side there are often provided separate measurement arrangements for the output voltage and output current, and the signal generated by these arrangements is transmitted as a response or feedback signal to the primary side through a opto-coupler or a small signal transformer.
In many applications, both the output voltage and the output current have a given allowed maximum value, so that the fixed power adjustment is not sufficient as such. By using adjusting systems that are based on measurements carried out on the secondary side, the output features of the power supply are easily restricted to fall under given maximum values, but the opto-coupler or signal transformer needed for conducting the response signal increases both the structural complexity of the power supply and the production costs. In the prior art there also are known systems where the adjusting arrangements provided on the primary side of the switched-mode power supply restrict the operation of the device, so that the output voltage and output current do not surpass the allowed maximum values. For instance, the U.S. Pat. No. 4,172,276 deals with a switched-mode power supply where the isolation transformer contains, in addition to the primary coil and secondary coil, also a third coil that generates a given auxiliary voltage to the primary side. The switching impulses of the switching transistor that keeps interrupting the primary current is adjusted by a reference voltage, generated by certain condensers and a zener diode, where the problem is particularly the dependence of the potential difference prevailing across the zener diode for the current passing therethrough. Among possible solutions, said U.S. Patent suggests the coupling of the fixed current supply in parallel to the zener diode, but the invention proper specified in the application is based on the fact that in order to generate a corrected reference voltage, into the voltage prevailing across the zener diode, there is added a compensating voltage which is formed according to the measured primary current. Thus the primary side includes a current measuring circuit. On the other hand, there also is used an additional coil for detecting the waveform of the secondary voltage and for imitating it on the primary side. The original reference voltage that feeds current through the zener diode at the switching transistor base is formed exactly by means of this additional coil.
From the Finnish patent application No. FI 952,571, there is known a system that adjusts the output voltage and output current of a switched-mode power supply in an extremely versatile and accurate manner on the basis of the operation of the primary side arrangements only. In the system of said application, an additional coil included in the isolation transformer forms an image voltage that in waveform corresponds to the secondary voltage. The image voltage is corrected by means of various correcting terms that compensate its distortions while the output features of the switched-mode power supply change. The pulse ratio of the switching pulses of the switching transistor is determined on the basis of the corrected image voltage. The switched-mode power supply according to said invention is meant to be used particularly in battery chargers, where the output current must be maintained nearly constant while the output voltage rises from a value corresponding to the terminal voltage of an empty battery to a value corresponding to the terminal voltage of a full battery, and thereafter the output voltage must remain below a given maximum value, although the fully charged battery does not receive a considerable amount of current anymore. The system according to said publication fulfils its objectives to a satisfactory degree, but is relatively complicated in structure, which increases the production costs.